Variations of climate, frontal zones and recruitment to major
commercial fish stocks in the Barents Sea
Oleg TITOV, Vladimir Ozhigin 2005
Knipovich Polar Research Institute of Marine Fisheries and Oceanography (PINRO) 6 Knipovich Street, 183763 Murmansk Russia
e-mail: [email protected]
Atlantic waters
Barents Sea waters Arctic waters
coastal waters
The Barents Sea is characterized by a good water exchange with the North Atlantic and Arctic oceans and has a number of water masses with different features. Interaction of these water masses makes quite a vivid picture of frontal zones in the sea. It is thought that owing to interaction between the boreal and Arctic waters, the ecosystem of the sea is noted for a high
Water masses distribution (Loeng, 1991). Feeding areas and spawning grounds of cod and capelin.
Introduction
biological production and is rich in organisms important for the fisheries.
Capelin, the pelagic species, migrate for feeding to the cold Arctic and Barents Sea waters but spawn in the warm coastal waters of the North Norway. Northeast Arctic cod, the demersal species, feed and spawn in the warm coastal and Atlantic waters. Both cod and capelin distribution varies depending on climate conditions and related to frontal zones in the periods of feeding and wintering.
Until recently, there have been no scientific publications on interannual variability in parameters of thermal frontal zones in the Barents Sea as well as on the effect of such variability on biological and fisheries productivity of the sea.
Nowadays there is a following hypothesis (Titov, 2001). The largest increase of horizontal temperature gradients in the frontal zones occurs in the periods with relatively extensive ice coverage in the Barents Sea while heat advection by Atlantic currents is getting stronger. An index indicating sharpening of the Barents Sea frontal zones based on the Barents Sea ice coverage and temperature in the upper 200 m layer of the Kola Section was suggested.
An increase in this index coincides in time with a decrease in oxygen content in the bottom layer in the Kola Section that may be a consequence of higher biological productivity in the photic layer and settling of organic matter to the bottom. Relationships between variations in the above index and strength of capelin and cod yearclasses were found to be significant.
On the whole, sharpening of the frontal zones was perceived as an indicator of interaction strength between the Arctic and boreal oceanic systems.
The purpose of this paper is to estimate year-to-year variability in characteristics of the frontal zones under the effect of climate fluctuations and to show the relationship between such variability and biological and fisheries productivity of the Barents Sea.
Introduction
The study is based on temperature data at the surface, 50 and 100 m, and in the bottom layer in July–November 1951-2003 (about 20 000 stations).
In July-November the ice edge is located to the north of the Polar front, which makes it possible to get a correct estimation of the frontal zones characteristics.
Material
Time series of averaged (July–November)
monthly anomalies of
water temperature (upper 200m layer),
oxygen saturation (bottom layer) in the
Kola section and ice coverage in the Barents Sea were used.
The anomalies were normalized by dividing the averaged anomalies by relevant standard
deviations. Ice coverage in different months (PINRO data) and Kola section locations
70°N
75°N
80°N
10°E
20°E
30°E 40°E 50°E
60°E
70°E
OceanDataView
II IV
VI X
XII
IX XI
Kola section
Based on normalized temperature and ice coverage anomalies, the years (1951-2003) were divided into 4 groups:
1) WARM-years that is warmer-than-normal with decreased ice coverage;
2) COLD-years that is colder-than-normal with increased ice coverage;
3) WIIC-years that is warmer-than-normal with increased ice coverage;
4) CDIC-years that is colder-than-normal with decreased ice coverage.
Methods
This grouping was implemented by calculating sums and differences of normalized temperature and ice coverage anomalies that gave two new time series. The WIIC group is comprised by the years in which temperature exceeded its “balance” value typical at a certain ice coverage values. It means that ice coverage in the Barents Sea was larger than normally observed at certain thermal condition.
-3 -2 -1 0 1 2 3
-3 -2 -1 0 1 2 3
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
-3 -2 -1 0 1 2 3
-3 -2 -1 0 1 2 3
1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000
Water temperature (T)
Ice coverage (IC)
+
-
-
+
T - IC
T + IC
WARM COLD
CDIC WIIC
Years Years
Normalized anomalies
Methods
Initial temperature data was also divided into 4 groups according to the selected types of years.
Since data coverage in some areas of the Barents Sea was not good enough both in space and time, the sea area was divided into “squares” of about 60x60 miles. Only those “squares” that had at least 100 observations were used to calculate absolute values of horizontal temperature gradients.
Areas in the Barents Sea that have at least 100 measurements of temperature in “square” of 60x60 miles.
Density of observations on water temperature in the Barents Sea in July- November of WARM-years
WARM WIIC
CDIC COLD
Results
Average horizontal gradients of temperature (°C/km) in the bottom layer in years that differ in climatic conditions
Results
The highest gradients for all
groups of years were typical of 50 and 100 m depths
and difference between year types
is barely visible.
At the surface and in the bottom layer
gradients were considerably lower.
The most sharpened frontal
zones at the surface were typical of WIIC- and COLD- years and in the bottom layer of WARM- and WIIC- years.
Temperature gradients (°C/km) at different depths averaged over the study area and years that differ in climatic conditions
0,0100
0,0113
0,0100
0,0110
0,0095 0,0105 0,0115 0,0125 0,0135
WARM WIIC CDIC COLD surface
0,0141 0,0142 0,0141 0,0140
0,0095 0,0105 0,0115 0,0125 0,0135
WARM WIIC CDIC COLD
50 m
0,0135 0,0138
0,0134 0,0133
0,0095 0,0105 0,0115 0,0125 0,0135
WARM WIIC CDIC COLD
100 m
0,0116 0,0115
0,0105 0,0104 0,0095
0,0105 0,0115 0,0125 0,0135
WARM WIIC CDIC COLD
bottom
Results/Discussion
Presumable cause-and-effect relationships in the Barents Sea ecosystem
Yearclasses of the most important pelagic (capelin) and demersal (cod) fishes
350 400 450 500 550 600 650 700 750
COLD WIIC WARM CDIC
Climatic conditions Cod, 3+, 10 mln spp
100 150 200 250 300
Capelin, 1+, mlrd spp Temperature gradient in the upper 100 m
and bottom layer
0.01030.01080.0113
COLD WIIC WARM CDIC
Gradient, bottom, 0 С/км 0.01270.0132 Gradient, 0-100м, 0 С/км
Climatic conditions
Oxygen deficiency in the bottom layer -0,8
-0,6 -0,4 -0,2 0 0,2 0,4 0,6 0,8
COLD WIIC WARM CDIC
Climatic conditions
Anomaly, % of saturation
Ratio "temperature in the Kola section (0-100 m) and ice coverage of the sea"
-1.2-0.600.61.2
COLD WIIC WARM CDIC
T+C, conventional unit
Climatic conditions
Frontal zones
Recruitment Primary production/Settling of
organic matter to the bottom Climate/Interaction of
oceanic systems
Discussion
Idealized scheme of the relationships between climate changes, environmental conditions, settling of organic matter and strength of cod and capelin yearclasses
in the Barents Sea ecosystem
Capelin
Cod
Capelin t0
Cod
Capelin
Cod Capelin
t
0Cod
Capelin
Cod Capelin Cod
Cod
Capelin t0
Cod
Ice Ice Ice Ice
t
0Capelin
COLD WIIC WARM CDIC
Capelin
Capelin
Effect of the Arcticand boreal oceanic systems
Sharpening of frontal zones
Ice Ice coverage
t0 Intensity of heat advection
Intensity of organic matter settling to the bottom
Favourable/Unfavourable conditions in the
spawningand nursery areas
